Glass breakage detectors have been used for many years as part of intruder alarm systems some of which, for example, include a piezo-electric transducer detection element for sensing when a glass, such as a glass window, either cracks or breaks. The piezo-electric detector element is normally affixed directly to the surface of the glass and senses the presence of a mechanical wave signal which is a specific harmonic of the ultrasonic frequency mechanical wave signals which are transmitted through the glass when it fractures. The detector must normally be in a location on a glass window, for example, which is sufficiently removed from the frame in which the glass is encased in order to reduce the damping which may be imposed on the signal to be detected because of the presence of the frame structure and in order to reduce signals that may be reflected from the frame structure to the detector element. On the other hand, it is desirable that the detector be installed relatively near the frame for appearance's sake.
Accordingly, an installer must specifically determine the distance for such a desired minimum offset from the frame and then measure such distance during installation. Alternatively, an installer may estimate such minimum distance, permitting some degree of error on the high side, in order to locate the detector at a suitable region of the window. In the former case, the installer requires appropriate tools and time for installation, while, in the latter case, if the detector is positioned too far from the frame, the installed detector will require the presence of an undesired extra length of wire across the glass, which exposed wire will not only be unsightly, but can also interfere with cleaning of the glass and may even tend to encourage tampering and vandalism.
It is desirable then to provide a glass breakage detection system which can easily be correctly positioned relative to the frame and which, at the same time, can provide a satisfactory appearance and eliminate the need for leaving a length of wire across the glass.
Further, in currently available glass breakage detection systems, testing can be performed by electronically generating a simulated glass breakage wave signal and applying the simulated signal to the detection system in order to insure that the detection system is functioning properly.
It is desirable that the system be arranged to be tested regularly over the course of time in a relatively easy manner, i.e., one which does not require the breaking of a glass sample or the use of simulated signal generation equipment. If the system permits a relatively easy test to be performed, it is more likely that a user will do so, particularly if testing requires no such glass destruction or no extra tools or excessive time for such purpose.